Process for preparing desulfurizing and denitrating agents

ABSTRACT

This invention relates to a preparation process of desulfurizing and denitrating agents for an off-gas produced upon combustion of a fuel such as coal or heavy oil. 
     The agent can be obtained by providing as first raw material(s) one or more materials capable of yielding calcium oxide and calcium sulfate, providing as second raw material(s) one or more materials capable of yielding silicon dioxide and aluminum oxide, mixing the first raw material(s) or a mixture of the first raw material(s) and either fractional or whole portion(s) of the second raw material(s) with water, subjecting the resultant aqueous mixture to wet-air aging at room temperature or to steam aging, and optionally subjecting the thus-aged mixture further to a heat treatment at a temperature of 30° C. or higher. 
     The desulfurizing and denitrating agents of this invention are prepared simply from readily-available raw materials such as cement, slag, coal ash, glass and fluidized-bed combustion ash and show high activity in both desulfurization and denitration. They are thus useful for the prevention of air pollution.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

This invention relates to a preparation process of desulfurizing and denitrating agents for an off-gas produced upon combustion of a fuel such as coal or heavy oil.

(b) Description of the Prior Art

Sulfur oxides, nitrogen oxides and other noxious gases, which are contained in an off-gas produced upon combustion of coal, heavy oil or other fuels, have been found to cause damage to buildings and structures and moreover to have an extremely serious effect not only on animals and plants but also on humans. Research has been conducted on the removal of the above-described materials from off-gases by means of desulfurization and denitration, leading to the development of various processes.

These desulfurization and/or denitration processes may be roughly divided into dry-type processes and wet-type processes. As dry-type desulfurization processes to which the present invention pertain, there have been known those shown in Table 1.

                                      TABLE 1                                      __________________________________________________________________________     Dry-Type Off-Gas Desulfurization Processes*                                    Manner of                                                                            Process                Reaction                                                                            Reaction                                                                             Recovered                              treatment                                                                            feature                                                                               Reactants                                                                             Process name                                                                            temp.                                                                               product                                                                              material                               __________________________________________________________________________     Absorption                                                                           Reacted with                                                                          Activated                                                                             Activated manga-                                                                        130° C.                                                                      MnSO.sub.4                                                                           Ammonium                                     a metal                                                                               manganese                                                                             nese oxide process  sulfate                                      oxide to                                                                              oxide                                                                   form sulfate                                                                          Alkalized                                                                             Alkalized alumina                                                                       280- Na.sub.2 SO.sub.4                                                                    Sulfur                                              alumina                                                                               process  360° C.                                                 Limestone,                                                                            Lime blowing                                                                            500- CaSO.sub.4,                                                                          Mixture of                                          slaked lime                                                                           process  1200° C.                                                                     etc.  unreacted lime                                      dolomite or                oxide and                                           dolomite                   gypsum                                              hydroxide                                                         Adsorption                                                                           Adsorption of                                                                         (Adsorbent)                                                                           Activated carbon                                                                        100° C.                                                                      H.sub.2 SO.sub.4                                                                     Sulfuric acid                                SO.sub.2 with an                                                                      Activated                                                                             process                                                          adsorbent                                                                             carbon Reinluft process                                                                        100° C.                                                                      SO.sub.2                                                                             Sulfur                                 Catalytic                                                                            Oxidation of                                                                          (Catalyst)                                                                            Benelec process                                                                         420- SO.sub.3                                                                             Sulfuric acid                          oxidation                                                                            SO.sub.2 into SO.sub.3                                                                Vanadium-                                                                             (for coal boilers)                                                                      480° C.                                          by a catalyst                                                                         base   TIT-Kiyoura                                                                             220- (NH.sub.4).sub.2 SO.sub.4                                                            Ammonium sulfate,                                   catalyst                                                                              process  260° C.                                                                            sulfuric acid                          __________________________________________________________________________      *Cited from the following publication:                                         "Smoke and Soot Control Technology Series, No. 7 Smoke and Soot Control        Technology for Electric Power Industry", Government of Japan, Ministry of      Industrial Science and Technology, Enterprise Bureau and Public Benefit        Utilities Bureau, (1969).                                                

The above-described conventional dry-type desulfurization processes however involve a variety of problems. In the absorption processes shown in Table 1, it is required, for example, to use costly NH₃ (the activated manganese oxide process) or a valuable reducing gas (the alkalized alumina process) for the regeneration of the reactants (and for the recovery of sulfur or sulfur compounds) or to employ a high reaction temperature (the alkalized alumina and lime blowing processes). The adsorption processes are accompanied by such drawbacks as having to use expensive activated carbon, which is susceptible to deterioration. In the catalytic oxidation processes, it is necessary to use expensive vanadium-base catalysts, which, moreover, are liable to deterioration, and relatively-high reaction temperatures are required.

It is the catalytic reduction process making use of ammonia that is widely used as a dry-type denitration process. This process is however accompanied by such problems as having to rely upon an expensive catalyst and having a high running cost because ammonia is oxidized by nitrogen gas and hence consumed completely.

Further, no practical process which permits simultaneous execution of desulfurization and denitration in a dry state has materialized.

SUMMARY OF THE INVENTION

An object of this invention is to solve the above-described various problems of conventional dry-type desulfurization and denitration processes and to provide desulfurizing and denitrating agents which permit simultaneous execution of desulfurization and denitration. This object has been achieved by the following processes:

A process for preparing desulfurizing and denitrating agents, which comprises providing as first raw material(s) one or more materials capable of yielding calcium oxide and calcium sulfate, providing as second raw material(s) one or more materials capable of yielding silicon dioxide and aluminum oxide, mixing the first raw material(s) or a mixture of the first raw material(s) and either fractional or whole portion(s) of the second raw material(s) with water, and then subjecting the resultant aqueous mixture to wet-air aging at room temperature or to steam aging.

A process for preparing desulfurizing and denitrating agents, which comprises providing as first raw material(s) one or more materials capable of yielding calcium oxide and calcium sulfate, providing as second raw material(s) one or more materials capable of yielding silicon dioxide and aluminum oxide, mixing the first raw material(s) or a mixture of the first raw material(s) and either fractional or whole portion(s) of the second raw materials with water, subjecting the resultant aqueous mixture to wet-air aging at room temperature or to steam aging, and then subjecting the thus-aged mixture to a heat treatment at a temperature of 30° C. or higher.

The desulfurizing and denitrating agents prepared in accordance with the above-described processes of this invention are different from conventional absorbents and adsorbents and are composed of one or more materials capable of yielding CaO and CaSO₄ and optionally one or more additional materials capable of yielding SiO₂ and Al₂ O₃. As their raw materials may thus be used silicic acid salts or calcium compounds such as cement, slag, glass, coal ash or fluidized-bed combustion ash. Accordingly, usable raw materials may be chosen from a wide range of materials. Furthermore, the desulfurizing and denitrating agents of this invention can be prepared by the simple step of wet-air aging or steam aging.

Since the desulfurizing and denitrating agents prepared in accordance with the above-described processes of this invention permit effective reutilization of waste materials such as coal ash, slag, glass and fluidized-bed combustion ash, they are useful as a technique for recycling such waste materials as resources and in addition, they permit simultaneous and efficient execution of desulfurization and denitration. Therefore, the desulfurizing and denitrating agents have brought about a significant contribution to the prevention of pollution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing one example of comparison in performance between a desulfurizing and denitrating agent obtained in accordance with the latter process of this invention and conventional adsorbents; and

FIG. 2 is a graph showing performances of other desulfurizing and denitrating agents obtained in accordance with the latter process of this invention.

DETAILED DESCRIPTION OF THE INVENTION

As exemplary materials capable of yielding calcium oxide and useful in the practice of this invention may be mentioned quick lime, slaked lime, lime carbonate, cement, slag and dolomite plaster (with lime contained therein).

Illustrative of the material capable of yielding calcium sulfate may include gypsum dihydrate, gypsum hemihydrate, quick lime, slaked lime, a combination of lime carbonate and sulfuric acid and calcium sulfite.

Illustrative of the material capable of yielding silicon dioxide may embrace silica, metasilicic acid, aluminum silicate and calcium silicate as well as compounds containing reactive silicon dioxide, such as cristobalite, tridymite, kaolin, bentonite, talc, perlite, shirasu (i.e., white volcanic ash similar to acid glass or pumice in nature and its secondary deposit, which is available abundantly and widely in the southern part of Kyushu, Japan), diatomaceous earth and glass.

As illustrative examples of the material capable of yielding aluminum oxide may be mentioned compounds containing reactive aluminum, such as alumina, aluminum hydroxide, aluminum silicate, alumina sulfate, alum, aluminum sulfide, aluminum sulfate, aluminum chloride, bentonite, kaolin, diatomaceous earth, zeolite and perlite.

Furthermore, as other exemplary materials each of which can yield two or more of the above-mentioned four types of compounds may be mentioned coal ash (source for calcium oxide, silicon dioxide and aluminum oxide), cement (source for calcium oxide, calcium sulfate, silicon dioxide and aluminum oxide), slag, and minerals containing reactive silicon dioxide, aluminum, calcium and/or similar minerals, such as shirasu, andesite, chert, quartz trachyte, opal, zeolite, feldspar, clay minerals and ettringite, as well as furnace desulfurization ash such as fluidized-bed combustion ash and desulfurization byproducts obtained by stack gas desulfurization.

The present inventor combined materials which can yield the above-described four types of compounds (calcium oxide, calcium sulfate, silicon dioxide and aluminum oxide, hereinafter referred to as "base materials") in various ways, and then mixed the resultant mixture with water in order to harden them. The thus-obtained samples were tested to determine their desulfurizing and denitrating performances. As a result, it was found that they give unexpectedly-good performances depending on the combination of the base materials and their preparation process, leading to completion of this invention.

In the present invention, one or more materials capable of yielding calcium oxide and calcium sulfate are called "first raw material(s)". It should however be borne in mind that sufficient desulfurization and denitration effects can still be brought about by a combination of these base materials only. Calcium oxide and calcium sulfate, which are yielded, show satisfactory desulfurization and denitration effects over a wide ratio of 1:99 to 99:1. It is however preferred to control the proportions of calcium oxide and calcium sulfate at 80:20.

The desulfurizing and denitrating performance can be improved further by combining one or more materials which yield silicon dioxide and/or aluminum oxide and are called "second raw material(s)" in the present invention, with the above-described first raw material(s).

The proportions of the base materials are:

    ______________________________________                                                CaO         at least 1%                                                        CaSO.sub.4  at least 0.1%                                                      SiO.sub.2   0-90%                                                              Al.sub.2 O.sub.3                                                                           0-70%.                                                      ______________________________________                                    

The following proportions are however preferred:

    ______________________________________                                                CaO    1%-80%                                                                  CaSO.sub.4                                                                           0.1%-70%                                                                 SiO.sub.2                                                                             5-90%                                                                   Al.sub.2 O.sub.3                                                                       5-70%.                                                          ______________________________________                                    

Raw materials are ground if needed, followed by proportioning. Water is added further and is then mixed with the above-ground and proportioned raw materials. If one of the combined raw materials is, for example, dilute sulfuric acid and sufficient water is contained in the dilute sulfuric acid, it is unneccessary to add water subsequent to their proportioning. It is suitable to add water in an amount of about 20 parts to about 50 parts per 100 parts of dry raw materials.

The resultant mixture is then subjected to wet-air aging at room temperature or to steam aging. In the case of wet-air aging, it is preferred to conduct it for about 1 week at a relative humidity of 50-100%. Steam aging may preferably be carried out at a temperature of 60° C.-100° C. and a relative humidity of 100% for 5-72 hours.

The thus-aged and hardened mixture is ground or granulated and classified by usual methods to obtain the desulfurizing and denitrating agent of this invention.

Its desulfurizing and denitrating performance can be improved further by subjecting the above-aged and hardened mixture to a heat treatment at 30° C. or higher, preferably, within a temperature range of 50° C.-200° C. for 1-10 hours.

The present invention will hereinafter be described in further detail by the following Examples.

EXAMPLES 1-11

Commercial slaked lime, calcium sulfate, silicic acid, aluminum hydroxide, calcium silicate and aluminum silicate were proportioned and mixed in accordance with compositions shown in Table 2. Water was added to the resulting mixtures, followed by further mixing. The thus-prepared mixtures were subjected to steam aging at 100° C. for 24 hours, and the resultant hardened mixtures were ground and classified to 0.5-0.7 mm in particle size. They were heat-treated at 130° C. for 2 hours to obtain desulfurizing and denitrating agents of this invention. The percentages of base materials CaO, CaSO₄, SiO₂ and Al₂ O₃ in each of the agents (hereinafter called "composition of base materials") are given along with results of their performance tests in Table 4.

As their performance tests, desulfurization tests (SO₂ removing tests) were carried out at a gas temperature of 130° C. under the test conditions given in Table 3. Measurement data of their specific surface areas are also shown in the same table. Measurement of their specific surface areas was effected in accordance with the BET method subsequent to degasfication of the samples at 200° C. for 1 hour.

                                      TABLE 2                                      __________________________________________________________________________     Proportions of Raw Materials                                                   Raw Material                                                                                           Calcium                                                                             Aluminum                                          Example                                                                             Ca(OH).sub.2                                                                        CaSO.sub.4.2H.sub.2 O                                                                 CaSO.sub.4.1/2H.sub.2 O                                                               silicate                                                                            silicate                                                                             H.sub.2 SiO.sub.3                                                                  Al(OH).sub.3                            __________________________________________________________________________     1    27.9%                                                                               45.7%  --     26.4%                                                                               --    --  --                                      2    27.9%                                                                               58.9%  --     --   13.2% --  --                                      3    90%  --     10%    --   --    --  --                                      4    80%  --     20%    --   --    --  --                                      5    60%  --     40%    --   --    --  --                                      6    55%  --     40%      5% --    --  --                                      7    55%  --     40%    --     5%  --  --                                      8    55%  --     40%     2.5%                                                                                2.5% --  --                                      9    27%  63%    --     --   --    --  10%                                     10   24%  56%    --     --   --    11% 9%                                      11   22%  50%    --     --   --    20% 8%                                      __________________________________________________________________________

                  TABLE 3                                                          ______________________________________                                         Composition of model gas used                                                                       Amount                                                    ______________________________________                                         SO.sub.2 (ppm)       450                                                       NO.sub.x (ppm)       230                                                       O.sub.2 (%)           6                                                        CO.sub.2 (%)          13                                                       H.sub.2 O (%)         10                                                       N.sub.2 (%)          Balance                                                   Gas volume rate of flow (l/hr)                                                                       60                                                       Amount of packed desulfurizing                                                                       6                                                        and denitrating agent (ml)                                                     Space velocity (SV) (hr.sup.-1)                                                                     10,000                                                    Reaction temperature (°C.)                                                                   130                                                       ______________________________________                                    

                  TABLE 4                                                          ______________________________________                                                                             Specific                                                             Rate of SO                                                                               surface                                    Exam- Composition of base material (%)                                                                   removal   area                                       ple   CaO     CaSO.sub.4                                                                             SiO.sub.2                                                                            Al.sub.2 O.sub.3                                                                      (%)    (m/g)                                ______________________________________                                         1     26      42      32    0     84      20                                    2*   33      67      0     20    77      16                                   3     88      12      0     0     51       8                                   4     76      24      0     0     62      10                                   5     55      45      0     0     39      14                                    6*   55      39      6     0     43      13                                    7*   55      39      0     6     64      14                                    8*   55      39      3     3     74      14                                   9     25      58      0     17    29       3                                   10    23      52      11    14    99      11                                   11    21      47      19    13    100     45                                   ______________________________________                                          *SiO.sub.2 and Al.sub.2 O.sub.3 are shown in terms of calcium silicate an      aluminum silicate, respectively.                                         

EXAMPLES 12-16

To portions of coal ash having the composition shown in Table 5 and containing SiO₂ and Al₂ O₃ at high levels, dilute sulfuric acid solutions having concentrations of 1-12 wt. % were added in the weight proportions given in Table 6. After treating the coal ash portions with the corresponding dilute sulfuric acid solutions at room temperature for 1-2 days, slaked lime (Ca(OH)₂) was added in accordance with Table 6. The resultant mixtures were subjected to steam aging in the same manner as in Examples 1-11 to harden them. They were thereafter ground and classified to 0.5-0.7 mm in grain size. The thus-ground and classified samples were dried at 130° C. for 2 hours to obtain the desulfurizing and denitrating agents of this invention.

Proportions of the base materials in each of the agents are shown in Table 7, along with results of their performance tests.

Test conditions were the same as those employed in Examples 1-11. The tests were each carried out at gas temperatures of 50° C., 80° C. and 120° C. Measurement data of their specific surface areas are also given in the same table. In addition, time-dependent variations in denitrating and desulfurizing performance of the sample of Example 16 at 80° C. are shown in FIG. 1, together with results obtained with the samples (activated carbon and silica gel) of the Comparative Examples which will be described later. For the sake of comparison, similar tests were also conducted on samples which had each been obtained by calcining three types of coal ash samples at 1,000° C. They however showed substantially no desulfurizing and denitrating effects.

                  TABLE 5                                                          ______________________________________                                         Chemical Composition of Coal Ash                                               ______________________________________                                                Ignition loss                                                                           1.0%                                                                  SiO.sub.2                                                                               60%                                                                   Al.sub.2 O.sub.3                                                                        23%                                                                   CaO      4%                                                                    Fe.sub.2 O.sub.3                                                                        5%                                                                    MgO      2%                                                                    Na.sub.2 O                                                                              0.7%                                                                  K.sub.2 O                                                                               1.3%                                                                  TiO.sub.2                                                                               1.0%                                                                  Others   2.0%                                                           ______________________________________                                    

                  TABLE 6                                                          ______________________________________                                                 Material                                                                         Coal         Slaked  Sulfuric                                        Example   ash          lime    acid*                                           ______________________________________                                         12        80.5%        19%     0.5%                                            13        80%          19%     1%                                              14        79%          19%     2%                                              15        77%          19%     4%                                              16        75%          19%     6%                                              ______________________________________                                          *In terms of 100% sulfuric acid                                          

                                      TABLE 7                                      __________________________________________________________________________                                       Specific                                     Proportions of base materials (%)                                                                   Rate of SO.sub.2 removal (%)                                                                surface                                      Example                                                                             CaO CaSO.sub.4                                                                         SiO.sub.2                                                                          Al.sub.2 O.sub.3                                                                   50° C.                                                                      80° C.                                                                       120° C.                                                                     area (m/g)                                   __________________________________________________________________________     12   18.0                                                                               0.6 51  19.4                                                                               90  75   65  13                                           13   17.9                                                                               1.2 51  19.4                                                                               96  86   74  14                                           14   17.4                                                                               2.2 50.3                                                                               19.3                                                                               96  89   80  25                                           15   16.3                                                                               4.5 49.5                                                                               19.0                                                                               96  94   82  61                                           16   15.2                                                                               6.9 48.7                                                                               18.7                                                                               98  96   87  45                                           __________________________________________________________________________

EXAMPLES 17-20

To portions of coal ash of the same type as that employed in Examples 12-16, calcium sulfate dihydrate and slaked lime were mixed in the weight proportions given in Table 8. The resultant mixtures were subjected to steam aging, grinding, classification and drying in the same manner as in Examples 12-16 to obtain the desulfurizing and denitrating agents of this invention.

                  TABLE 8                                                          ______________________________________                                                 Material                                                                         Coal         Slaked  Calcium                                         Example   ash          lime    sulfate*                                        ______________________________________                                         17        80.7%        18.6%   0.7%                                            18        80.4%        18.2%   1.4%                                            19        79.4%        17.5%   2.8%                                            20        77.0%        15.0%   8.0%                                            ______________________________________                                          *Percentage in terms of CaSO.sub.4 in the dihydrate.                     

Proportions of the base materials in each of the agents, results of their performance tests conducted under the same test conditions as those employed in Examples 12-16 and their measurement results are shown in Table 9. In addition, time-dependent variations in denitrating and desulfurizing performance of the samples of Examples 19 and 20 at 80° C. are shown in FIG. 2.

                                      TABLE 9                                      __________________________________________________________________________                                       Specific                                     Proportions of base materials (%)                                                                   Rate of SO.sub.2 removal (%)                                                                surface                                      Example                                                                             CaO CaSO.sub.4                                                                         SiO.sub.2                                                                          Al.sub.2 O.sub.3                                                                   50° C.                                                                      80° C.                                                                       120° C.                                                                     area (m/g)                                   __________________________________________________________________________     17   18.1                                                                               0.7 50.7                                                                               19.5                                                                               95  83   72  12                                           18   17.8                                                                               1.5 50.4                                                                               19.4                                                                               90  79   68  17                                           19   17.2                                                                               2.9 49.9                                                                               19.2                                                                               94  90   78  19                                           20   15.0                                                                               8.3 47.9                                                                               18.4                                                                               100 100  100 49                                           __________________________________________________________________________

EXAMPLES 21-23

To portions of coal ash of the same type as that employed in Examples 12-16, portland cement of a composition given in Table 10 was mixed in the weight proportions given in Table 11, followed by the addition of water and subsequent mixing. The resultant mixtures were subjected to steam aging, grinding, classification and drying in the same manner as in Examples 12-16 to obtain the desulfurizing and denitrating agents of this invention.

                  TABLE 10                                                         ______________________________________                                         Chemical Composition of Portland Cement (wt. %)                                ______________________________________                                                Ignition loss                                                                           1.6%                                                                  SiO.sub.2                                                                               21.2%                                                                 Al.sub.2 O.sub.3                                                                        5.4%                                                                  CaO      63.1%                                                                 Fe.sub.2 O.sub.3                                                                        4.2%                                                                  MgO      1.1%                                                                  CaSO.sub.4                                                                              2.7%                                                                  Na.sub.2 O                                                                              0.4%                                                                  K.sub.2 O                                                                               0.1%                                                                  Others   0.2%                                                           ______________________________________                                    

                  TABLE 11                                                         ______________________________________                                                    Material                                                            Example      Coal ash Portland cement                                          ______________________________________                                         21           20%      80%                                                      22           40%      60%                                                      23           80%      20%                                                      ______________________________________                                    

Proportions of the base materials in each of the desulfurizing and denitrating agents, results of their performance tests conducted under the same test conditions as those employed in Examples 12-16 and their measurement results are shown in Table 12. In addition, time-dependent variations in denitrating and desulfurizing performance of the sample of Example 23 at 80° C. are also shown in FIG. 2.

                                      TABLE 12                                     __________________________________________________________________________                                       Specific                                     Proportions of base materials (%)                                                                   Rate of SO.sub.2 removal (%)                                                                surface                                      Example                                                                             CaO CaSO.sub.4                                                                         SiO.sub.2                                                                          Al.sub.2 O.sub.3                                                                   50° C.                                                                      80° C.                                                                       120° C.                                                                     area (m/g)                                   __________________________________________________________________________     21   51.3                                                                               2.2 29.0                                                                                8.9                                                                               55  40   28  4                                            22   39.5                                                                               1.6 26.7                                                                               12.4                                                                               50  37   20  5                                            23   15.8                                                                               0.5 52.2                                                                               19.5                                                                               89  85   74  7                                            __________________________________________________________________________

COMPARATIVE EXAMPLE 1

Performance tests were carried out under the same conditions as those used in Examples 12-16 except that commercial activated carbon, which was used for gas chromatography, and commercial crushed silica gel desiccant were each classified to grain sizes of 0.5-0.7 mm and the SV was changed to 2,400 hr⁻¹. The specific surface areas of the activated carbon and crushed silica gel were also measured. Test and measurement results are shown in Table 13. In addition, time-dependent variations in in denitrating and desulfurizing performance of the activated carbon and crushed silica gel at 80° C. are also shown in FIG. 1.

                  TABLE 13                                                         ______________________________________                                         Performances of Activated Carbon, Etc.                                                                  Specific                                                       Rate of SO.sub.2 removal (%)                                                                   surface                                               Sample     50° C.                                                                          80° C.                                                                            120° C.                                                                       area (m/g)                                  ______________________________________                                         Activated  76      70        47    1,080                                       carbon                                                                         Silica     77      56        21      530                                       gel                                                                            ______________________________________                                    

EXAMPLE 24

This Example describes results of a performance test of the desulfurizing and denitrating agent (grain size: 3-10 mm) obtained in the above-described Example 20 in Table 8, on a combustion off-gas from an operating boiler.

                  TABLE 14                                                         ______________________________________                                         Composition of off-gas                                                                               Amounts                                                  ______________________________________                                         SO.sub.2 (ppm)        355-520                                                  NO.sub.x (ppm)        232-315                                                  O.sub.2 (%)           7.6-7.9                                                  CO.sub.2 (%)          11.0-11.6                                                H.sub.2 O (%)          8.2-10.6                                                N.sub.2 (%)           Balance                                                  Dust (mg/Nm.sup.3)    200-940                                                  Gas volume rate of flow (Nm.sup.3 /hr)                                                               50                                                       Gas temperature (°C.)                                                                         132                                                      Amount of packed desulfurizing                                                                       64                                                       and denitrating agent (kg)                                                     Packing density (kg/l)                                                                               0.64                                                     ______________________________________                                    

As to performance, the desulfurization rate was maintained at 100% for 77 hours, at 90% and higher for 94 hours and at 20% and higher for 94 hours. In the performance test, SO₂ was absorbed in an amount of 100 mg or more per gram of the desulfurizing and denitrating agent. It was thus confirmed that almost 100% of the calcium contained in the desulfurizing and denitrating agent was caused to react with SO₂. Accordingly, the desulfurizing and denitrating agent of this example had the surprising feature that each grain reacts with its central part although the reaction is a gas-solid interfacial reaction.

EXAMPLES 25-26

The water content of an aged desulfurizing and denitrating agent obtained in accordance with the composition of Example 20 was adjusted as shown in Table 15 by controlling the degree of its heat treatment.

                  TABLE 15                                                         ______________________________________                                         Proportions of        Water content in                                         used materials (%)    desulfurizing and                                                Coal   Slaked    Calcium                                                                               denitrating agent                              Ex.     ash    lime      sulfate*                                                                              (%)                                            ______________________________________                                         25      77     15        8      25**                                           26      77     15        8      5***                                           20      77     15        8       0****                                         ______________________________________                                          *The dihydrate was used. The amounts used are shown in terms of                CaSO.sub.4.                                                                    **Not heattreated.                                                             ***Dried at 100° C.                                                     ****Heattreated at 130° C. for 2 hours.                           

Proportions of the base materials in each of the agents, results of their performance tests conducted under the same test conditions as those employed in Examples 12-16 and their measurement results are shown in Table 16.

                  TABLE 16                                                         ______________________________________                                                                   Rate of SO                                                                               Specific                                                             removal   surface                                    Exam- Composition of base material (%)*                                                                  (%)       area                                       ple   CaO     CaSO.sub.4                                                                             SiO.sub.2   130° C.                                                                         (m/g)                                ______________________________________                                         25    15.0    8.3     47.9  18.4  40      49                                   26    15.0    8.3     47.9  18.4  70      49                                   20    15.0    8.3     47.9  18.4  100     49                                   ______________________________________                                          *The analysis data are all on a dry basis.                               

The desulfurizing and denitrating agent (grain size: 3-10 mm) of Example 25 was tested under the same test conditions as those employed in Example 24. The maintenance time of 100% desulfurization rate was 8 hours. A comparison with the corresponding data of the desulfurizing and denitrating agent of Example 24 readily shows the significant effects of the heat treatment. 

I claim:
 1. A process for preparing a desulfurizing and denitrating agent, which comprises providing as first raw material(s) one or more materials capable of yielding calcium oxide and calcium sulfate, providing as second raw material(s) one or more materials capable of yielding silicon dioxide and aluminum oxide, mixing the first raw material(s) or a mixture of the first raw material(s) and either fractional or whole portion(s) of the second raw material(s) with water, and then subjecting the resultant aqueous mixture to wet-air aging at room temperature or to steam aging.
 2. A process for preparing a desulfurizing and denitrating agent, which comprises providing as first raw material(s) one or mor materials capable of yielding calcium oxide and calcium sulfate, providing as second raw material(s) one or more materials capable of yielding silicon dioxide and aluminum oxide, mixing the first raw material(s) or a mixture of the first raw material(s) and either fractional or whole portion(s) of the second raw materials with water, subjecting the resultant aqueous mixture to wet-air aging at room temperature or to steam aging, and then subjecting the thus-aged mixture to a heat treatment at a temperature of 30° C. or higher. 